Elucidating the Redox Behavior in Different P‑type Layered Oxides for Sodium-Ion Batteries

Sodium layered transition-metal oxides have attracted great attention for advanced Na-ion batteries (NIBs) because of their rich structural diversity and superior specific capacity provided by not only cation redox reactions but also possible oxygen-related anionic redox reactions. However, they usually undergo severe electrochemical performance fading, especially the voltage retention during the cationic and anionic redox processes. Herein, we design and synthesize a couple of novel sodium lithium magnesium aluminum manganese oxides (Na_0.75Li_0.2Mg_0.05Al_0.05Mn_0.7O₂) with the same Na⁺ coordination environment but different oxide layer stacking sequences, namely, P2-NLMAMO and P3-NLMAMO. We systematically investigate and compare the voltage decay phenomenon and the cationic/anionic redox processes under different electrochemical cycling windows combined with ex situ hard and soft X-ray absorption spectroscopy techniques. The results clearly indicate that the P2-NLMAMO electrode with a lower extent of Mn redox is prone to deliver a superior capacity retention and rate performance, more importantly, a higher average voltage in contrast to the P3-type counterpart. In addition, negligible change is detected for the average discharge voltage upon extended cycling when increasing the discharge cutoff voltage to 2.5 V for both P2-NLMAMO and P3-NLMAMO. This unique feature work provides an effective strategy for developing high-capacity P-type layered cathodes based on both cationic and anionic redox chemistry under controlled crystal structure arrangement, which could lead to a deeper understanding of the correlation between crystal structure and electrochemical performance for NIBs.